A primary application for the Controlled Fluid Transfer System will be as a Wearable Non-Gravity-Dependent Infusion Device to supply a predetermined volume of infusate at a selectable constant flow rate, without need for or disturbance due to elevation, depression or position of the IV fluid container. This will provide a substantial advantage during the movement or transport of persons for whom continuous IV infusion must be maintained, as well as for ambulatory patients. The term "wearable" signifies miniaturization to a degree that will permit the IV fluid container and the device to be carried by an ambulatory patient with a minimum of discomfort, while allowing freedom of action. The size and weight of the system will be limited by those of the container and amount of IV fluid that is carried, and the infusion device will be a minor part of the total (e.g. 25% to 40%).
Consideration of a non-gravity-dependent intravenous infusion device was originally applied to a military requirement for equipment to "deliver measured amounts of fluids at a constant reliable rate without technical supervision--for mass casualty and transport of wounded." In such situations it would be necessary to infuse drug or nutrient solutions over a period of hours in crowded areas without benefit of medical assistance. Examples of such conditions would be in evacuation aircraft or vessels, transporting a large number of individuals. What is desired is the capability to place the IV fluid container next to the individual, on the individual or on a litter, in any position or orientation, and maintain the required constant flow rate. The value of a wearable system is evident here, since it permits the individual receiving infusion to move or to be moved, with no need to detach and move the IV fluid container and infusion device separately.
It is also appreciated that there are comparable requirements for a wearable, or miniature, non-gravity-dependent infusion device for civilian applications, particularly where there is transport or movement of patients or injured individuals receiving continuous infusion. Examples are:
a) Intra-hospital transport of patients, as for periodic transport between the medical intensive care unit and a diagnostic scan facility, or from the surgical intensive care unit to the patient's room. Availability of a miniature infusion device that can be secured to the patient's bed (or the patient directly) should ease the task of the transport nurse.
b) Infusion during diagnostic procedures where rate must be controlled in different (e.g. inverted) positions (anesthesia during air contrast pneumoencephalography).
c) Transport of patients between hospitals or treatment centers.
d) Emergency transport of seriously injured individuals from remote or inaccessible locations, as by a litter secured to a helicopter. A wearable infusion device attached directly to the individual is advantageous, as it will no longer be a factor in moving the injured person.
e) Evacuation of injured from the scenes of disasters, such as fires, explosions, storms, earthquakes. The situations could be similar to the military conditions discussed above.
f) Total parenteral nutrition. For patients who require continuous parenteral infusion of nutrients in a hospital setting, and who otherwise could be ambulatory, a wearable infusion device would provide freedom from attachment to immobile equipment. For the majority of patients who undergo parenteral feeding at home, the time of infusion of the TPN solution is between 8 and 14 hours, mostly at night. Where the required duration is greater than the sleep period, a wearable infusion device will be advantageous, as it will permit normal movement and activity for the remainder of the infusion time.
In present clinical applications, where infusion with gravity lines and manual flow clamps is inadequate, an infusion pump or controller will be used. Controllers, in effect, automate the procedure normally followed with manual, gravity-fed administration sets. Usually, the drop rate of IV fluid into the drop chamber is sensed photo-optically, and an IV line clamp is automatically adjusted to maintain the drop rate constant at the selected value. The actual flow rate depends not only on the drop rate but also on the drop size, which varies with the viscosity and surface tension of the particular fluid being administered, and the type of fluid must be taken into account for an accurate determination of flow rate. The maximum flow obtained with a controller is the same as that of a simple gravity feed; and the flow rate is sensitive to back pressure and container height.
Pumps can provide the desired flow rate without requiring a gravity head for the IV supply; they also can pump against a much higher back pressure that could be required by small-pore filters or intra-arterial infusions. Pumps are usually of the peristaltic types or reciprocating displacement types with replaceable cassettes (Abbott, IMED, IVAC). Although current infusion pumps are of the displacement type, there is still an effect of gravity head on flow rate, which varies depending on the pump. Also, the weights of commercial pumps are relatively high, and the sizes and weights of these pumps are disadvantages for their use in non-gravity-dependent field or wearable applications.
In accordance with the teachings of the new Controlled Fluid Transfer System, a Non-Gravity-Dependent Infusion Device will include a pressurizable reservoir to contain the (IV) fluid and means to measure and adjust the flow of the fluid by a closed loop control, including pressurization of the reservoir, to provide a selected value of flow rate.
Trapped air in commercial infusion bags and bottles, which are only partially filled with infusion fluid, can present a problem for their use in a pressurized, non-gravity-dependent infusion system. In conventional gravity-feed infusion procedures, the fluid container is hung with the outlet at the bottom, and the air retained in the container rises to the top, where it remains during the procedure. The liquid in the fluid transfer line is under positive pressure, and there is little danger of the trapped air entering the line. In a non-gravity-dependent infusion system, however, the fluid filled container could be placed in any position or orientation and the trapped air could just as well collect at the container outlet as elsewhere. Means must be provided, therefore, to remove any air that enters the line from the container, either by an initial air removal procedure, use of an in-line air trap, or both. An air trap should be included in any event to remove small air bubbles that cling to the walls of the container and that could enter the transfer line. Also, since there is some prospect of an air bubble traveling down the fluid transfer line to the needle, an air-in-line detector should be provided, as is required for information pumps, in which air could leak into the system due to negative pressure at the pump inlet.
A number of infusion devices have been described in which the IV fluid is delivered from a flexible container at a purportedly constant flow rate through pressurization of the container.
In U.S. Pat. No. 3,640,277 to Adelberg, the driven fluid is retained in a container having a collapsible volume which is pressurized by a driving fluid. The driving fluid is administered from a pressure regulator through a selectable flow restrictor, which determines the flow rate of the driven fluid.
Olson in U.S. Pat. No. 4,337,769 shows a liquid administration device in which the liquid is contained in a flexible, collapsible bag, which is located in a housing filled with compressed cellular material. The cellular material exerts pressure on the collapsible bag to drive the liquid, whose flow rate is governed by a valve.
U.S. Pat. No. 4,857,055 to Wang shows a compression device in which a flexible solution container is positioned evenly between two inflatable sacs within a rigid casing. The sacs can be inflated by an aerosol to provide constant pressurization of the container, depending on temperature. Selectable capillary flow moderators in the delivery line determine the solution flow rate.
In a Portable Infusion Device shown in U.S. Pat. No. 5,059,182 to Laing, an air pressure bladder is in intimate contact with a flexible solution reservoir within the housing. The air pressure bladder has a volume at least five times that of the reservoir. A flow restrictor determines the flow rate of the solution. The increase in air volume in the bladder due to discharge of the solution from the reservoir is limited in relation to the original volume, limiting the decline in air pressure and the change in flow rate during delivery.
All of these prior devices use open loop systems, in which the fluid is forced through a restrictor by more or less constant pressurization of a flexible fluid reservoir. There is no simple way of adjusting flow rate to accommodate different needle resistances except by prior calibration to determine pressurization level, where this is adjustable, or restrictor size. The devices count on low venous pressures, in which changes will not greatly affect the overall pressure drop. But the devices are not suitable for arterial infusion where the pressure levels are elevated and variable.
Also, in none of the prior art devices described above, is there any recognition of the potential problem presented by air retained in the fluid container, or any illustration or discussion of air removal procedures or devices, or air-in-line detectors.